Palaeoclimate and Climate Modelling with the New CSIRO Coupled
Atmosphere-Ocean-Landsurface-Sea-Ice Scheme

Modelling the global climatic system is recognised as a computational
science `grand challenge'. In general, palaeoclimate modelling has so far used
global climate models (GCMs) that are restricted in their representation of
ocean-atmosphere-sea ice-land interactions than those now entering use for
modelling present-day and future climates. In Australia, CSIRO has developed
an advanced family of GCMs (that include ocean dynamics) which have been shown
to simulate present climate as accurately as the best overseas GCMs.

Our project is aimed at verifying that the CSIRO models are also world-leading
palaeoclimatic models by simulating past climates which are extensively
documented from geological and palaeoclimatic data, such as the initiation and
the climax of the last ice age. The project has three strands - testing model
sensitivity to boundary conditions such as CO2 and seasonal solar radiation,
simulation of selected past climates, and international intercomparison with
other models.

We are using two model schemes, a lower resolution scheme known as CSIRO4 which
is coupled to a dynamic upper ocean model, and CSIRO9 which has higher
resolution and is optionally coupled to a full ocean circulation model. We run
CSIRO4 on the CSIRO Division of Atmospheric Research (DAR) Cray supercomputer.
CSIRO9 is now established on the ANU VP2200 supercomputer. In 1994 we
completed a major series of sensitivity experiments with CSIRO4, including
intercomparison with the US National Centre for Atmospheric Research CCMl
model. This successfully simulated present climate and we are completing a
simulation of climate of 6000 years ago.

What are the basic questions addressed?

We address the general objective of testing the CSIRO GCMs against
palaeoclimatic data through a set of questions that are of high international
interest and have not been resolved by modelling groups elsewhere.

Can ice age initiation be simulated with appropriate orbital forcing? Does the
phase of climate change in the southern hemisphere relative to the northern
hemisphere change with intensity of orbital forcing and of CO2 forcing? Can
deep ocean-climate interactions that occurred during the ice ages be
simulated?

Are results from the above experiments robust or sensitive to internal and
external model parameters?

What are the results to date and the future of the work?

We have developed a coupled atmosphere (CSIRO4)/dynamical upper ocean
climate model, the first of its kind, with comprehensive sea ice - soil water -
cloud and other submodels, for palaeoclimate studies. In the course of this
work we also examined the sensitivity behaviour of the coupled model with
variations of solar constant, orbital forcing and a wide range of CO2, and
compared the sensitivity and signal/noise behaviour of our model with the
widely used North American CCMl climate model.

During the last six months the CSIRO9 climate model with vertical resolutions
of 9 levels and R42 resolution in the horizontal domain (approx. 1.6x2.8 deg)
was integrated on the VP2200 for 11 years of present day climate conditions.
This represents the control case against which a range of perturbation
experiments will be compared. A second 11-year run now being executed on
VP2200 represents the case of the Holocene climatic optimum of 6000 years ago.
(This is part of our contribution to the international Palaeoclimate Modelling
Intercomparison Project (PMIP) -- Presentation of our simulations and
comparisons with other model results will take place in early October in
France, during the second PMIP workshop.) Both of these runs used fixed sea
surface temperatures. At the same time, the modelling group at CSIRO DAR
completed highly successful integration of the full atmosphere-ocean-sea
ice-biosphere model for transient CO2 doubling and tripling.

We now propose a series of experiments, shared between the VP2200 and the DAR
Cray. Because of the large compute time required for the coupled CSIRO9/full
ocean scheme, we propose only one, carefully selected experiment to be run on
the VP2200 that will require 520 hours of cpu time. This 21, 000 year
experiment (climax of last ice age) will be the first to simulate full ice age
climate with fully coupled and tested model scheme. By comparison with well
established palaeo data, the predictive ability and performance of the deep
ocean circulation model under highly altered climatic conditions can be
ascertained. Complementary control experiments of shorter duration will be run
on the Cray.

In the longer term, we foreshadow using the fully coupled model with transient
boundary conditions to simulate initiation of the last ice age (116,000 years
ago). Our previous modelling with the lower resolution CSIRO4 with coupled
dynamic ocean has indicated icesheet initiation and monsoon changes compatible
with palaeo data; this has not been well simulated by teams overseas and is a
major test for climate models.

What computational techniques are used and why is a supercomputer
required?

A combination of spectral and finite difference techniques are used to
solve both three-dimensional atmospheric dynamics and ocean circulation as well
as coupling. The computational complexity of the coupled model, the essential
high resolution and the time-scales of global climatic behaviour all conspire
to place this problem beyond the reach of all but the largest supercomputers.